The transpiring among aged volcanic rocks, the cavities usually are stuffed with deposits of secondary minerals launched by percolating drinking water. The glass itself kinds threads, fibres, and skinny partitions among the vesicles. Rhyolite and trachyte pumices are white, andesite pumices typically yellow or brown, and pumiceous basalts (for instance https://edenproductsltd.co.uk/2025/04/17/the-history-of-pumice-stone-from-ancient-civilisations-to-modern-day-uses/